1. Field of the Invention.
This invention relates to a self-equilibrating measuring balance for example for use in weighing articles or for monitoring varying pressures.
2. Description of the Prior Art.
For measuring water movements and water level changes, for instance in the sea or in waterways, use is conventionally made of a pressure sensor arranged at the measuring point, and comprising a cell closed by an elastically-deformable wall, which cell is connected by a first duct to a source of compressed air with a constant air supply, and which by means of a second duct communicates with the ambient air, a pressure gauge being, furthermore, connected to the supply duct. The pressure measured by this pressure gauge is correlated with the flow resistance caused by the cell, which, in turn, is correlated with the deflection of the deformable wall, and, therefore, with the pressure at the location of the sensor. In this manner it is possible remotely to measure pressures, and more particularly continuously varying pressures, with high sensitivity. The pressure variations correlated to water levels and water movements have to be recorded over very long periods so that recording instruments are used. Since, generally, the recording instruments are positioned in remote places in which often no electricity supply is available, these instruments should be as simple and reliable as possible, and should be provided with individual energy sources so as to be anble to work without maintenance for a relatively long time. Therefore simple mechanical recording instruments, for instance with a clockwork drive, and simple reliable pressure gauges are preferred.
For such purposes it has been proposed to use a selfequilibrating balance comprising a balance arm supported on a fulcrum and provided, at one end, with a fixed mass, a pressure sensing cell suitable for sensing the pressure being connected to said arm. Furthermore, a travelling mass is mounted on this balance arm, which is movable by means of a screwed spindle driven by an electric motor, the latter being controlled by means of a circuit comprising two contacts co-operating with a contact finger at one end of the balance arm, so as to cause the motor to rotate in a sense such that the travelling mass is displaced so as to restore the equilibrium of the balance. This screwed spindle is coupled to a mechanical recording instrument for recording the pressure measured.
Such a pressure measuring mechanism operates reliably, the electrical motor for displacing the travelling mass needing very little current, and thus being able to run for an adequate length of time on batteries. Together with a mechanical recorder with a clock-work drive a very reliable unit is obtained, which can operate for a relatively long period without maintenance. At intervals the batteries are to be replaced and the clock-work is to be wound, but this can be done when the paper-roll with te pressure record is taken away for processing.
Previously proposed measuring balances, however, appear to have, at a pressure of for instance 10 to 20 m. of water, a measuring error of about 1 to 2%, i.e., about 100 to 200 mm. of water, which inaccuracy is too large for some purposes. This inaccuracy is, perhaps, a consequence of the fact that a very accurate adjustment of the travelling weight moved along a rod is not possible because of the friction and inertia of the weight, and of the unavoidable lost motion in the driving members acting on the weight, and possibly also as a consequence of friction between these driving members and the mass which influence the latter's vertical movements. The spring reaction of the contact springs can also be a disturbing factor promoting overshooting of the equilibrium position, and the restoration thereof is delayed or prevented by the aforementioned factors.
It is an object of the invention to provide a selfequilibrating measuring balance which has a substantial improvement in its accuracy in comparison with previously proposed balances.